The present invention is related to electronic circuits, and more particularly to a charging circuit for charging deeply discharged cells.
Many portable electronic devices utilize a rechargeable battery to provide power. These devices include computers, cellular telephones, pagers, radios, and the like. While there are many types of rechargeable batteries used today, including nickel cadmium and nickel metal hydride, lithium ion batteries have become a popular choice. Lithium ion batteries are typically smaller and lighter than other rechargeable battery types while charge capacity is increased.
The charging of lithium ion batteries is conducted in a different manner than the charging of nickel type rechargeable batteries. Generally, nickel type rechargeable batteries are charged by applying a constant current until the cell reaches a predetermined voltage or temperature. A lithium ion cell, however, uses a different charging process. First, the lithium ion cell is supplied with a current until the cell""s voltage rises above a threshold. Next, the battery charger is held at the threshold until the current of the cell decreases to a predetermined level.
These battery charging circuits typically have difficulty charging deeply discharged cells. Instead, the battery charging circuits only operate on cells that are not deeply discharged.
The invention is directed at providing a battery charging circuit for deeply discharged cells. According to one aspect of the invention, a deeply discharged charging circuit is used to charge the cell until the cell""s voltage reaches a predetermined threshold.
According to another aspect of the invention, a primary charging circuit is used to charge the cell when the cell""s voltage is above the predetermined threshold. The primary charging circuit regulates the charging of the cell in response to the charger and cell""s current charging condition.
According to another aspect of the invention, at a very low voltage of the cell, a resistive limited current is provided to the cell through the sense resistor and a diode.
According to another aspect of the invention, a POR circuit is used to enable and disable portions of the primary charging circuit when the cell is deeply discharged and the deeply discharged charging circuit when the cell is not deeply discharged.
According to another aspect of the invention, a method is directed at charging a cell that may be deeply discharged. When the cell is deeply discharged, a deeply discharged charging circuit is used to charge the cell. When the cell is not deeply discharged, a primary charging circuit is used to charge the cell.